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Calculating VA and VAr from data sheets

Calculating VA and VAr from data sheets

Calculating VA and VAr from data sheets

(OP)
Chaps,

Is it possible to calculate the VA (apparent power) and the VAr (reactive power) of a three phase induction motor by using information from the manufactures data sheets?

I've tried it but must be doing something wrong because when I plot these values on a power triangle I'm getting a ridiculous value for cos phi giving me a power factor of 0.4 which is bonkers. The formulas I'm using give me VA=401, VAr=395 all for a motor of 180W (yes this is a very tiny motor)

Ta

RE: Calculating VA and VAr from data sheets

That'd be tough I think. You also have to consider that a motor is rated by its shaft power first, not it's electrical power. So your motor is rated for 180W shaft power. The motor's efficiency (and PF, voltage, actual load) will determine what your consumption numbers are going to be. Difficult to predict.

RE: Calculating VA and VAr from data sheets

Quote:

The formulas I'm using give me VA=401, VAr=395
From that we could calculate Pin = sqrt(401^2-395^2)=69w.
Calculated Pin = 69w < Pout = 180w at rated conditions.

It seems like something's wrong with your calculation somewhere.

What is the data you're starting with and what are you trying to calculate?

=====================================
(2B)+(2B)' ?

RE: Calculating VA and VAr from data sheets

(OP)
I've sorted it folks. gotta use input power and not output power. Recalculating gives me Input real power = 305W (not 180W as on nameplate), apparent power = 401.8VA and reactive power = 261.6VAr. Now when plotted I get an angle of around 40 degrees which is 0.76 power factor. Great, thought I was going stupid.

Thanks for your inputs

RE: Calculating VA and VAr from data sheets

As WWWJD already said, the input apparent power of 401 VA has to be multiplied by two factors: efficiency [ approximate 0.6 for this motor] and cosfi [power factor] of 0.7 approx.
So , the active input power-includes the shaft output power and the losses: copper, magnetic, windage and friction -has to be 180/0.6=300 W.

RE: Calculating VA and VAr from data sheets

Small motors often have poor efficiency and poor PF.
PF is determined from input power and input reactive power.
As a result, paradoxically, the poor efficiency improves the power factor.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: Calculating VA and VAr from data sheets

I'm curious how you calculated the VA without already knowing the power factor.

305/402 = 0.76 or 76% for power factor.

Efficiency is only 59%?


There are the only formulas you need to calculate motor W, VA and VAR values.
Efficiency = Pout/Pin
PF = W/VA
and
Pythagoras theorem VA^2 = W^2 + VAR^2

RE: Calculating VA and VAr from data sheets

What information does the data sheet give for this motor?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

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